Metal-oxide-semiconductor field-effect transistor (MOSFET) may include vertical double-diffused MOSFET and lateral double-diffused MOSFET. Comparing to the vertical double-diffused MOSFET, the lateral double-diffused MOSFET has many more advantages, including better thermal stability and frequency stability, higher gain and durability, lower feedback capacitance and thermal resistance and constant input impedance, and simpler biasing circuit.
FIG. 1 depicts a conventional N-type LDMOS transistor. The conventional N-type LDMOS transistor includes a semiconductor substrate (not illustrated), a P-well 100 in the semiconductor substrate, an N-type drift region 101, and a STI structure 104 in the N-type drift region 101. The STI structure 104 is used to increase the conduction path of the LDMOS transistor to increase breakdown voltage of the LDMOS transistor. The conventional N-type LDMOS transistor also includes a gate 105 formed on the semiconductor substrate. The gate 105 is formed cross over the P-well and the N-type drift region 101 and is partially located on the STI structure 104. The conventional N-type LDMOS transistor also includes an N-doped source region 102 in the P-well and an N-doped drain region 103 in the N-type drift region. The N-doped source region 102 and N-doped drain region 103 are formed on both sides of the gate 105.
However, it is desirable to improve isolation performance between the LDMOS transistor and other semiconductor devices, and also between the LDMOS transistor and the semiconductor substrate.